1. Field of the Invention
The present invention relates to an optical sensor for detecting a specific component such as ions in a sample by an optical change due to a potential-sensitive dye which is fixed in an ultra-thin membrane. It also relates to a specific substance detecting device comprising such an optical sensor. The sensor of the present invention is referred to as an optical sensor, since it detects a specific substance based on optical changes.
2. Description of the Related Art
The detection of a biopotential by using a potential-sensitive dye has been successfully developed, and its usefulness has been demonstrated, as described in J. Membrane Biol., vol. 19, pages 1-36 (1974), and Kagaku (Science) vol. 55, No. 5 (1985), pages 290-299. The former document, FIG. 4 on page 15, shows the result of experiments in which potential-sensitive dyes are diffused into a living cell, tissue, or the like, and proves that there is a linear relationship, within a certain range, between change in membrane potential and optical change of absorbance or fluorescence. Numerous kinds of dyes are known as potential-sensitive dyes and many examples are listed in the former document, pages 28-33.
However, these dyes do not have a function of identifying or selecting a specific substance such as an ion, and are sensitive only to a potential. Since each type of dye has its own response speed, sensitivity, durability, and the like, the most suitable one should be selected for a particular purpose. Further, as described in Ann. NY. Acad. Sci., vol. 33, page 217-241 (1977), Table 2 on page 223 in particular, the sensitivities of these dyes are all about 1.times.10.sup.5 V/cm, and are effective, in fact, only in a thin membrane of the submicron order, such as a cell membrane.
Since preparation of an ultra-thin membrane, as mentioned above, under moderate conditions has up to now been extremely difficult, use of the above potential-sensitive dyes are limited mainly to studies of biomembrane functions. The only exception to this is Published Unexamined Japanese Patent Application (PUJPA) No. 61-241656, which discloses an electrolyte concentration measuring sensor. This sensor comprises a polymer layer in which ion-selective substances are dispersed, and a potential-sensitive dye fixed on the surface of the polymer layer. The fluorescent intensity varies in accordance with changes in ion concentration in a sample solution. According to the technique stated in this PUJPA, the layer or film is basically prepared by a coating method, and it is extremely difficult to prepare a minute or dense ultra-thin membrane which is free from defects. Consequently, a sensor made with such a membrane may be ineffective in terms of sensitivity and response speed.
In the meantime, the Langmuir-Blodgett (LB) film/membrane preparation technique has been developed, allowing a variety of thin membranes to be designed. The LB membrane preparation technique is explained in detail in Bunshi Sekkei Gijutsu (Molecular Designing Technique) edited by Masakazu OKADA et al. (published by Science Forum Co., Ltd, Japan), pages 89 to 101. An amphipathic or amphiphilic substance is used as the material for the LB membrane. Stearic acid, .omega.- tricosenic acid, and the like, generally used as materials for forming the LB membrane, have carboxyl groups. These carboxyl groups are ionically dissociated to have a negative charge in an aqueous solution. It has been found by the present inventors that when an LB membrane is made of these carboxylic acids and the potential-sensitive dyes are embedded therein, ionic components in a sample solution and the negatively-charged carboxylate ions bring about non-specific coupling and dissociation therefrom, thereby creating a problem in terms of sensitivity and stability.